An anti-ACVRI antibody exacerbates heterotopic ossification by fibro-adipogenic progenitors in fibrodysplasia ossificans progressiva mice

• Published in: The Journal of clinical investigation 2022-06, Vol.132 (12)
• Main Authors: Lees-Shepard, John B, Stoessel, Sean J, Chandler, Julian T, Bouchard, Keith, Bento, Patricia, Apuzzo, Lorraine N, Devarakonda, Parvathi M, Hunter, Jeffrey W, Goldhamer, David J
• Format: Article
• Language: English
• Subjects: Autoantibodies; Bone morphogenetic proteins; Connective tissue diseases; Development and progression; Genetic aspects; Growth factor receptors; Health aspects; Physiological aspects
• ISSN: 0021-9738; 1558-8238
• DOI: 10.1172/JCI153795

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disease characterized by progressive and catastrophic heterotopic ossification (HO) of skeletal muscle and associated soft tissues. FOP is caused by dominantly acting mutations in the gene encoding the bone morphogenetic protein (BMP) type I receptor, ACVR1 (ALK2), the most prevalent of which results in an arginine to histidine substitution at position 206 (ACVR1[R206H]). The fundamental pathological consequence of FOPcausing ACVR1 receptor mutations is to enable activin A to initiate canonical BMP signaling in fibro-adipogenic progenitors (FAPs), which drives HO. We developed a monoclonal blocking antibody (JAB0505) against the extracellular domain of ACVR1 and tested its effect on HO in 2 independent FOP mouse models. Although JAB0505 inhibited BMP-dependent gene expression in wild-type and ACVR1(R206H)-overexpressing cell lines, JAB0505 treatment profoundly exacerbated injuryinduced HO. JAB0505-treated mice exhibited multiple, distinct foci of heterotopic lesions, suggesting an atypically broad anatomical domain of FAP recruitment to endochondral ossification. This was accompanied by dysregulated FAP population growth and an abnormally sustained immunological reaction following muscle injury. JAB0505 drove injury-induced HO in the absence of activin A, indicating that JAB0505 has receptor agonist activity. These data raise serious safety and efficacy concerns for the use of bivalent anti-ACVR1 antibodies to treat patients with FOP.